View Full Version : Ground glass focusing with fresnal screens
When focusing with a fresnal screen on a view camera, if I use frount lens tilt, I find it very hard to see detail at the edge of the glass. ( like the blades o f grass on the ground) This critical focusing is almost impossible with a fresna l lens. What can be done to remedy this difficulty? Must I replace the ground gl ass?
Bob Salomon - HP Marketing
A fresnel screen or an enhanced brightness system like a Beattie?
The problem you are seeing is due to the loupe or your eye not remaining on the optical axis. Tilt the loupe so it is on axis and you should see the corners. This effect can be worse with enhanced brightness screens and is worse with base tilt movements as compared to optical axis tilt movements
Why not try a Silvestri Tilting loupe !!! This is a high quality loupe that has an adapter that allows the body to tilt, allowing placement of the field of view right into the corners of the GG. It comes with a lanyard and eyepiece cover as well as a standard "skirt" that allows the loupe to be used as "normal". Here in the U.K.they are available from Robert White (www.robertwhite.co.uk)who sells/advertises through View Camera magazine. It isn't cheap, about #80 stirling, but it is very good !! Hope this helps. Paul
You have found the problem with fresnel lenses. They focus all the light from the ground glass inward, toward the center. All the light is aimed at one point. If your eye (keep one eye closed for best viewing) is in the center of the ground glass and at a certain distance from it, the entire viewing screen is bright and clear. If you more your view off center, which is what you do when you view the corners with a loupe, the glass with go dark or even black out. You can tilt the loupe so that you approach the corners from the center of the glass. When you use movements, like tilt, the glass can go dark or black out if the movements are extreme. What you describe is normal with fresnel viewing.
I used to use a fresnel lens, but I no longer do for the reasons you have described. A fresnel lens obscures detail when focusing. You will be able to focus better without a fresnel. I say, remove the fresnel and try a plain ground glass. You make like it better. I do.
Have you seen what R. Wisner has to say about fresnels. See his web site www.wisner.com, and look at the section that contains his articles. He goes into detail.
John: If you decide to remove the Fresnel lens, check to see if the lens is in front of the ground glass, as is the case in many of the cameras made during the past 40 years or so. If so, you will need to build a spacer the thickness of the Fresnel lens to put in front of the ground glass. Most Fresnel lenses are about 1/16 of an inch thick, and you can't move the ground glass forward that much without being out of focus. I made some spacers using strips off a cardboard sheet which is packaged with printing paper. It might not match to the 1,000th of an inch but it worked fine. Fresnel lenses are a a mixed blessing. As the man said, you don't get anything in large format without giving up something. Once you get used to it, a plain ground glass works great. Doug
The Silvestri loupe is available from B&H - haven't gotten mine yet, so I can't comment on the quality.
Robert A. Zeichner
One cautionary note about removing fresnels that reside between the gg and the lens: It is not a simple matter of replacing the fresnel with a stack of shims that equal the thickness of the fresnel. The fresnel is a lens! As such it displaces focus by a distance equal to approximately 1/3 the thickness of the fresnel. Simply replacing it with shims of equal thickness will place the gg out of alignment! Getting the ground glass aligned is very important to the performance of the camera. It's not enough to get it close. You really need to get it perfect! That way, the $30 plastic holders that by design are supposed to fall within a +/- 0.007" tolerance will all work with satisfactory results. Many cameras that were designed with fresnels are constructed in such a way that an existing set of shims just needs to be removed to place the gg in the proper plane. An example of this is the Horseman 45FA. The fresnel is clamped to the gg, but not sandwiched between it and the mounting pads. The shim stack is just the right thickness so that removal will automatically place the gg in the right plane when the fresnel is removed. A very clever design, if you ask me. Many who prefer fresnels like to put them on the rear of the gg where the focus shift is automatically compensated for by our vision. This way, you can remove the fresnel at will when focal length of lens makes it preferable to focus without it.
Robert is right. If the fresnel is behind the ground glass, you cannot easily remove it. The back will need modification. Bestwell does this kind of work. John, what camera do you have?
Fresnel lenses also have a focal length built into them. I have several on my Crown Graphics, and they appear to be approximately 135mm in focal length. This was determined by the distance between the fresnel and the sharp image of the sun. It makes sense, because the standard lens for the Graphics is the 135mm lens.
I have noticed the fresnel brightness distribution is significantly degraded when using lenses with a focal length shorter than that of the fresnel.
It would be most interesting to use Ed Romney's backsighting technique to gauge the difference in focal plane location between fresnel equipped backs, and that same back shimmed when the fresnel is removed. I find his backsighting technique to be quite sensitive to slight variations in focal plane distances.
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